Surface micro-planarization for enhanced optical efficiency and pixel performance in SLM devices

ABSTRACT

A method for enhancing the optical performance of a reflective spatial light modulator by micro-planarizing surfaces within the SLM, such as the reflective surface of each pixel, by gas-cluster-ion-beam bombardment.

This application claims priority under USC §119(e)(1) of provisionalapplication No. 60/255,857 filed Dec. 15, 2000.

FIELD OF THE INVENTION

The present invention relates to spatial light modulators andparticularly to reflective spatial light modulators used in projectiondisplay systems.

BACKGROUND OF THE INVENTION

Reflective spatial light modulators (SLMs) are small structurestypically fabricated on a semiconductor wafer using techniques such asoptical lithography, doping, metal sputtering, oxide deposition, andplasma etching, which have been developed for the fabrication ofintegrated circuits.

Two types of reflective SLMs include micromirror devices and reflectiveliquid crystal on silicon (LCD) devices. These devices use digital imagedata to modulate a beam of light by selectively reflecting portions ofthe beam of light on to a display screen.

In these devices the precision of the reflection angle is critical forgood optical efficiency. It is also critical that unwanted light not bereflected to the image where it would raise the black level and lowerthe image contrast. Thereby, a critical aspect in fabricating thesedevices is the smoothness of the reflective surface. For example, anystray light coming from the black areas of the image that gets into theprojected image degrades the system contrast ratio. As a result, it isdesirable to have extremely smooth reflective surfaces.

What is needed is a method to improve the reflectivity of a reflectivespatial light modulator.

SUMMARY OF THE INVENTION

The present invention discloses a method for enhancing the opticalperformance of a reflective spatial light modulator by micro-planarizingsurfaces within the SLM, including the reflective surface of each pixeland, in the case of micromirrors, any landing surfaces that have aneffect on the tilt angle of the micromirror.

The present invention inserts a gas-cluster-ion-beam bombardment at oneor more steps in the process. This process improvement has shown toimprove the surface smoothness from μ=10, σ=8 to μ=<3, σ=2.

By improving the smoothness of the reflective surfaces in thesereflective SLMs, which are primarily used in projection displays, thebrightness and constant ratio of the projectors can be significantlyenhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a small portion of a digital micromirrordevice with mirrors removed to show the underlying yoke/hinge andaddress/landing pad levels of the device.

FIG. 2 is an exploded perspective view of a single digital micromirrordevice pixel showing the substrate memory cell, address electrodes andmirror landing pad level, yoke/torsion hinge level, and reflectivemirror level.

FIG. 3 is a diagram illustrating the gas-cluster-ion-beam bombardmenttechnique, which is added at one or more steps in the improved SLMmanufacturing process flow of the present invention to micro-planarizeall reflective dependent surfaces within a SLM.

FIG. 4 is a diagram illustrating how a surface is smoothed using thegas-cluster-ion-beam bombardment micro-planarization technique.

FIG. 5 is a block diagram of the process flow, including thegas-cluster-ion-beam bombardment technique, for fabricating a digitalmicromirror device spatial light modulator using the improved process ofthe present invention.

FIGS. 6a and 6 b are graphs showing the mean and standard deviation of asurface being micro-planarized before and after the gas-cluster-ion-beambombardment technique in the improved process flow of the presentinvention.

FIG. 7 is a block diagram of a projection display system incorporating aspatial light modulator with enhanced optical efficiency as a result ofbeing micro-planarized using the gas-cluster-ion-beam bombardmenttechnique included in the improved SLM process flow of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Micromirror devices have small mirrors that tilt either ON or OFF(typically +10° or −10°), so that light reflected from the ON mirrors isprojected on to a viewing screen and light reflected from OFF mirrors isnot. These mirrors are attached to a yoke, which is extended above thesubstrate level of the device by torsion hinges connected to supportposts. The mirrors rotate in either the positive or negative directionuntil landing points on the yoke contact landing pads at a lower levelof the device, leaving the mirrors in either a binary ON or OFF state.It is important that both the mirrors and landing pad areas have verysmooth surfaces. Rough surfaces on the mirror will scatter incidentlight, lowering the image contrast. Rough landing surfaces can alter thelanding angle of the mirror and change the amount of light reflected bythe mirror to the image plane. Typical roughness of these micromirrorsurfaces is measured to be on the order of μ=10 nm (mean) and σ=8 nm(standard deviation).

The disclosed invention modifies the reflective spatial light modulatorfabrication process flow by adding a gas-cluster-ion-bombardment (GCIB)micro-planarization step at one or more steps in the process flow.

FIG. 1 shows a small portion of a micromirror, which is a digitalspatial light modulator (SLM) that often includes more than a thousandrows and columns of individual cells or pixels. The micromirror is onedevice in a family of devices known as micro-electromechanical systems(MEMS). FIG. 2 shows the build-up of a single micromirror cell. Thedevice is built-up in four levels, these being a memory substrate level10, an address electrode/landing pad level 11, a yoke/hinge level 12,and a mirror level 13, as indicated. The substrate 10 contains anorthogonal array of CMOS SRAM address circuits 100 over which thereflective micro-mirror superstructure is fabricated. A thick oxide 101isolation layer, which has vias 102 for connecting to the CMOS addresscircuits, is then placed on top of the CMOS array 100. This thick oxide101 surface is typically planarized using a chemical mechanicalpolishing (CMP) technique to provide a flat platform on which tofabricate the micromirror superstructure.

The mirror superstructure is then fabricated on top of this isolationlayer 101, beginning with an aluminum metal-3 layer 11, which includesyoke address electrodes 110,111 and landing pads 112. The addresselectrodes 110,111 connect to the respective binary outputs of the CMOSaddress circuits 100 through the vias 113 and vias 102 in the thickoxide layer. A voltage potential is applied between these address pads110/111 and the yoke 121 above the address electrode at level 12, whichsets up an electrostatic force that causes the yoke/mirror structure torotate on its torsion hinges 120.

The next layer 12 consists of the yoke 121 and torsion hinge 120structure, mirror address electrodes 123, and hinge posts 126 and postcaps 125. The yoke 121, which supports the mirror assembly 130 on thetop level 13, is suspended in air above the metal-3 layer 11 and rotatesabout a diagonal axis, on the torsion hinges 120, until the yoke landingtips 122 contact the landing pads 112 below. It is seen that thegeometry of the yoke 121 and the spacing between the metal-3 level 11and the yoke/hinge level 12 determines the tilt angle of the yoke/mirrorstructure. The hinge posts 126 sit on top of and in contact with themetal landing pads 112 at the metal-3 level 11, so that the yoke andlanding pads are at the same electrical potential. The mirror addresspads 123 are attached to the yoke addressing pads 110 by additionalposts 124. In this case, a voltage potential difference is appliedbetween the mirror address electrodes 123 and the mirror 130 above thepad at level 13, which sets up an electrostatic force that assists incausing the yoke/mirror structure to rotate on the torsion hinges 120.

The top level 13 consists of the reflective mirror 130 and mirror post131, which ride on top of the yoke 121. In operation, electrostaticforces cause the mirror/yoke structure to rotate on its torsion axis,defined along the torsion hinges. These electrostatic forces areestablished by the voltage potential difference between the yoke addresselectrode 110/111 and the yoke 121 and between the mirror addresselectrode 123 and the mirror 130, respectively. In each case, theseforces are a function of the reciprocal of the distance between the twoplates; e.g., 110/121 and 123/130. As the rigid yoke/mirror structurerotates on its axis, the torsion hinges 120 resist deformation with arestoring torque that is an approximately linear function of the angulardeflection of the structure. The structure rotates until either thisrestoring torsion beam torque equals the established electrostatictorque or until the yoke/mirror structure is mechanically limited in itsrotation, e.g., the yoke tips 122 lands on the landing pads 112. Fordigital display applications, the structure is such that it ismechanically limited in its rotation (lands), so as to provide stablestates at approximately +/−10°.

In any reflective SLM, the planarization, or smoothness of thereflective elements within the device, such as the reflective electrodesin a liquid crystal device and the mirror of a micromirror device, iscritical. This is both to assure that desired light gets into the focalplane and to prevent stray (unwanted) light from getting into the focalplane and degrading the brightness and contrast ratio of a projectionsystem.

The reflective quality of liquid crystal device address electrodes,whether formed on silicon, glass, or other substrate, is critical to theoptical efficiency of a display system. In a micromirror, the smoothnessof the yoke landing pads 112 is critical since it affects the precisionof the tilt angle which has a direct effect on the optical performanceof the device. Although the CMP step in the process flow, as discussedabove, provides a somewhat smooth isolated platform on which to buildthe micro-mirror superstructure, its smoothness is at least two ordersof magnitude too rough to avoid degradation of the system opticalefficiency.

One embodiment of the present invention involves micro-planarization ofsurfaces in the spatial light modulator using agas-cluster-ion-beam-bombardment planarization technique, as shown inFIG. 3. In a liquid crystal device, the address electrodes, whetherformed on silicon, glass, or other substrate, are smoothed using thedisclosed GCIB process. In a micromirror, mirror 130 and yoke landingpads 112 are smoothed.

In this process, a gas 30 is discharged through a nozzle 31 and thenloosely bound atoms or molecules 32 are ionized 33 by means of anelectrical discharge within the gas, producing gas cluster ions,consisting of many atoms or molecules weakly bound to each other andsharing a common electrical charge. It is well known that clusters ofatoms or molecules held together by weak inter-atom forces, know as vander Waals forces, can be formed by condensation occurring within theflow of a pressurized gas expanding from a small nozzle into a vacuum.These cluster ions are then propagated under vacuum and their energiesare controlled using acceleration voltages 35. The cluster ions 36 havemuch larger mass and momentum with lower energy per atom than a typicalmonomer ion carrying the same total energy and as a result, upon impactwith a solid surface produce an effect markedly different from monomerions.

FIG. 4 is a sketch illustrating the effect of micro-planarizing(smoothing) a solid surface 40 using the gas-cluster-ion-bombardmentprocess. Here the surface 40 is shown being bombarded with a cluster ion41, which has a combination of high total energy, mass and momentum butwith a correspondingly low energy per atom. This produces a virtualsimultaneous penetration of the target surface by a large number ofspatially coincident atoms or molecules. Unique to gas cluster ions isthe high rate of lateral sputtering 42 that occurs at the targetsurface, which smoothes solid surfaces. It is this effect thatsignificantly improves the smoothness of all reflective criticalsurfaces within the SLM.

FIG. 5 is a diagram of the improved process flow for the one embodimentof the present invention. In this embodiment, a micromirrorsuperstructure is monolithically fabricated over a CMOS 500 SRAM addresscircuit. Although this SRAM address circuit uses conventionalsemiconductor processing techniques, there are significant differencesdue to the mechanical nature of the superstructure to be built on top ofthe SRAM. The CMOS 500 circuit is basically an array of SRAM memorycells, which stores the binary state that causes each mirror to tilteither +10° or −10°.

The next step is to deposit a layer of thick oxide over the metal-2 ofthe CMOS array and then planarize this isolation layer using chemicalmechanical polishing (CMP) 501 techniques. Although this polishing orsmoothing process provides a flat substrate on which to build themechanical superstructure, its planarity is at least 2 orders ofmagnitude worse than what is needed at the critical reflective surfacesof the device.

The superstructure process begins by depositing a metal-3 layer 502 ofAluminum on top of the thick oxide substrate layer and then patteringand etching this Aluminum to form yoke address electrodes 110/111 andyoke landing pads 112, as discussed earlier.

Since the precision of the mirror tilt angle is critical to the opticalefficiency of a projection display, the smoothness of the mirror landingpads is important. Therefore, the process is modified at this point tomicro-planarize the metal-3 layer using the GCIB 503 technique discussedabove. Providing this micro-smooth surface for the tips of the mirrorsto land on significantly improves the optical performance of themicromirror by increasing both the brightness and the contrast of theprojected image.

An organic sacrificial layer is spun onto the micro-planarized surfaceof the metal-3 layer and then lithographically patterned and hardened,leaving vias 504 through this layer for metal support posts.

A hinge and yoke 505 (also called a beam) shown in magnified view 5050are formed. First, a thin metal layer, which is ultimately the hingematerial, is sputter-deposited on top of the sacrificial layer. Then alayer of SiO₂ is plasma-deposited over this thin metal layer andpatterned in the shape of the hinges 120. This pattern serves as an etchmask in the process. Next, a thicker layer of metal is sputter-depositedon top of the thin metal and SiO₂, where it is patterned andplasma-etched to form the yoke 121 attached to the hinges 120 and themirror address electrodes 123. In this structure the thicker metal yoke121 is attached to metal post 126 by the much thinner metal torsionhinges 120, so that when electrostatic forces are applied, the thinnerhinges tend to twist or torque, thereby allowing the thicker metal yoketo tilt.

A second sacrificial layer is then spin-coated onto the exposed yoke andhinge surface of the device and lithographically patterned and hardened,again leaving vias 506 for additional metal support post, this time inthe middle of the yoke 121 to support the mirrors 130.

The mirror metal 507 is sputter-coated on top of this second sacrificiallayer, also lining the support post via 506 holes. A layer of SiO₂ isthen plasma-deposited on top of the upper mirror metal surface where itis patterned and plasma-etched to form the individual mirrors 5070riding on top of the yoke 121, which is attached to the metal posts 126by thin torsion hinges 120.

A second micro-planarizing step is performed to smooth the exposed metalmirror 130 surfaces, once again using the GCIB 508 micro-planarizationtechnique.

The wafer of micromirror chips is then partially sawed 509 through. Thena plasma undercut 510 technique is used to remove the two sacrificiallayers from underneath the mirror 130 and yoke/hinge 121/120 structures,leaving the mirror assembly free to tilt in the positive or negativedirection, based on the binary state of the SRAM memory cell over whichit is built, when a voltage potential difference is applied.

Finally, the surface of the wafer is passivated 511 and the micromirrorsare tested, separated, and packaged 512 into individual spatial lightmodulators.

FIGS. 6a and 6 b are plots showing the results of smoothing the metalsurfaces using the GCIB technique described above. FIG. 6a is a plot ofthe mirror 130 and/or landing pad 112 metal smoothness before GCIBplanarization. A typical value for the mean and standard deviationbefore planarization (FIG. 6a) is μ=8.4 mm and σ=10.4 nm, respectively.After micro-planarization using the GCIB technique (FIG. 6b), themeasured values are μ=2.1 mm and σ=2.8 nm, respectively, indicatingsignificant improvement in surface smoothness.

Typically, in order to create an image using the digital micromirrordevice, the light source is positioned at an angle approximately equalto twice the angle of rotation of the mirrors so that mirrors rotatedtoward the light source reflect light in a direction normal to thesurface of the micro-mirrors and into the aperture of a projection lens,thereby creating bright pixels on an image plane (screen). On the otherhand, mirrors rotated away from the light source reflect light away fromthe projection lens, leaving the corresponding pixels dark. Intermediatebrightness levels are created by pulse width modulation techniques wherethe mirrors are rapidly and repetitively rotated on and off with binaryweighted periods. The duty cycle of the mirror determines the quantityof light reaching the screen. The human eye then integrates these lightpulses such that the brain perceives a corresponding brightness level.

Full color images are generated by using three micromirror devices toproduce three (red, green, blue) color images that are recombined or bysequentially forming three single color images using one digitalmicromirror device illuminated by a single beam of light passing througha rotating color filter wheel to produce the (red, green, blue) colorimages.

FIG. 7 is a schematic view of a digital projection system 70 using animproved SLM according to the present invention, to include themicro-planarization of critical surfaces within the device using theGCIB technique. As shown in FIG. 7, light from a light source 73 isfocused on to the improved SLM 71 by means of a condenser lens 74,placed in the path of the light. An electronic controller 72, isconnected to both the SLM 71 and the light source 73 and used tomodulate the SLM 71 and to control the light source 73. For all SLMpixels positioned towards the light source, the incoming light beam isreflected into the focal plane of a projection lens 75, where it resizedand projected on to a viewing screen 77 to form an image 78. On theother hand, SLM pixels positioned away from the light source 73, as wellas any stray light reflected from various near flat surfaces on andaround the SLM, are reflected into a dark trap 76 and discarded.

By modifying the process flow for fabricating SLM devices by including agas-cluster-ion-bombardment micro-planarizing step for all criticalsurfaces of the SLM, the optical efficiency (e.g., the system brightnessand contrast ratio) is significantly improved.

Thus, although there has been disclosed to this point a particularembodiment and method for surface micro-planarization for enhancedoptical efficiency and SLM pixel performance, it is not intended thatsuch specific references be considered as limitations upon the scope ofthis invention except insofar as set forth in the following claims.Furthermore, having described the invention in connection with certainspecific embodiments thereof, it is to be understood that furthermodifications may now suggest themselves to those skilled in the art, itis intended to cover all such modifications as fall within the scope ofthe appended claims.

What is claimed is:
 1. A method of fabricating a reflective spatiallight modulator, said method comprising: providing a substratesupporting a metal layer of a liquid crystal display; smoothing asurface of said metal layer using gas cluster ion beam bombardment; andcompleting said spatial light modulator.
 2. The method of claim 1 saidproviding comprising: providing a silicon substrate.
 3. The method ofclaim 1, said providing comprising: providing a glass substrate.